Planetary gear train of automatic transmission for vehicle

ABSTRACT

A planetary gear train of an automatic transmission for a vehicle may include an input shaft receiving torque, an output shaft disposed in parallel with and apart from the input shaft, a first planetary gear set having at least three rotation elements, a second planetary gear set having at least three rotation elements, a first rotation shaft, a second rotation shaft, a third rotation shaft, and a fourth rotation shaft.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to and the benefit of KoreanPatent Application No. 10-2013-0145221 filed Nov. 27, 2013, the entirecontents of which is incorporated herein for all purposes by thisreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automatic transmission for avehicle. More particularly, the present invention relates to a planetarygear train of an automatic transmission for a vehicle that can improvepower delivery performance and reduce fuel consumption at a low-speeddriving.

2. Description of Related Art

Typically, multiple-shift mechanism of an automatic transmission isachieved by combining a plurality of planetary gear sets and a pluralityof frictional elements. It is well known that when a planetary geartrain realizes a greater number of shift speeds, speed ratios of theplanetary gear train can be more optimally designed, and therefore avehicle can have economical fuel mileage and better performance. Forthat reason, the planetary gear train that is able to realize more shiftspeeds is under continuous investigation.

Though achieving the same number of speeds, the planetary gear train hasa different operating mechanism according to a connection betweenrotation elements (i.e., sun gear, planet carrier, and ring gear). Inaddition, the planetary gear train has different features such asdurability, power delivery efficiency, and size depending on the layoutthereof. Therefore, designs for a combining structure of a gear trainare also under continuous investigation.

If the number of shift-speeds, however, increases, the number ofcomponents in the automatic transmission also increases. Therefore,mountability, cost, weight and power delivery efficiency may bedeteriorated. Particularly, since the planetary gear train having anumber of components is hard to be mounted in a front wheel drivevehicle, researches for minimizing the number of components have beendeveloped.

The information disclosed in this Background of the Invention section isonly for enhancement of understanding of the general background of theinvention and should not be taken as an acknowledgement or any form ofsuggestion that this information forms the prior art already known to aperson skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing aplanetary gear train of an automatic transmission for a vehicle havingadvantages of achieving eight forward speeds and one reverse speed bydisposing first and second planetary gear sets on an output shaft andconnecting rotation elements of the first and second planetary gear setsto an input shaft through a plurality of transfer gears.

In addition, various aspects of the present invention are directed toproviding a planetary gear train of an automatic transmission for avehicle having further advantages of enabling of setting optimum gearratios due to ease of changing gear ratios by using the plurality oftransfer gears, and accordingly improving power delivery performance andfuel economy at a low-speed driving.

In an aspect of the present invention, a planetary gear train of anautomatic transmission for a vehicle may include an input shaftreceiving torque, an output shaft disposed in parallel with and apartfrom the input shaft, a first planetary gear set having three rotationelements, a second planetary gear set having three rotation elements,wherein one rotation element is integrally formed with or is directlyconnected to one rotation element of the first planetary gear set, afirst rotation shaft directly connected to the one rotation element ofthe first planetary gear set and/or the one rotation element of thesecond planetary gear set, and selectively connected to the input shaft,a second rotation shaft directly connected to another rotation elementof the first planetary gear set and another rotation element of thesecond planetary gear set, and directly connected to the output shaft, athird rotation shaft directly connected to the other rotation element ofthe first planetary gear set and selectively connected to the inputshaft or a transmission housing, and a fourth rotation shaft directlyconnected to the other rotation element of the second planetary gearset, and selectively connected to the input shaft through two paths orselectively connected to the transmission housing.

The planetary gear train may include three transfer gears disposedrespectively between the input shaft and the first and third rotationshafts and on the two paths.

The planetary gear train may further include a dog clutch selectivelytransmitting torque of the input shaft to any one of the two paths ornot transmitting either of the two paths.

Each of the first and second planetary gear sets is a single pinionplanetary gear set.

The first rotation shaft is connected to a first sun gear of the firstplanetary gear set and a second ring gear of the second planetary gearset, the second rotation shaft is connected to a first planet carrier ofthe first planetary gear set and a second planet carrier of the secondplanetary gear set, the third rotation shaft is connected to a firstring gear of the first planetary gear set, and the fourth rotation shaftis connected to a second sun gear of the second planetary gear set.

The three transfer gears may include a first transfer gear disposedbetween the input shaft and the first and third rotation shafts, asecond transfer gear disposed on one path of the two paths, and a thirdtransfer gear disposed on the other path of the two paths.

Gear ratios of the three transfer gears are different from each other.

The planetary gear train may further include a first clutch disposedbetween the first transfer gear and the first rotation shaft, a secondclutch disposed between the first transfer gear and the third rotationshaft, a first brake disposed between the third rotation shaft and thetransmission housing, and a second brake disposed between the fourthrotation shaft and the transmission housing.

The first planetary gear set is disposed at a radial exterior of thesecond planetary gear set.

The first, second, and third transfer gears, the first and secondclutches, and the second brake are disposed at one side of the first andsecond planetary gear sets, and the first brake is disposed at the otherside of the first planetary gear set.

In another aspect of the present invention, a planetary gear train of anautomatic transmission for a vehicle may include an input shaftreceiving torque, an output shaft disposed in parallel with and apartfrom the input shaft, a first planetary gear set including a first sungear, a first planet carrier, and a first ring gear as rotation elementsthereof, a second planetary gear set including a second sun gear, asecond planet carrier, a second ring gear as rotation elements thereof,wherein the first sun gear and the second ring gear are integrallyformed with or are directly connected to each other, a first rotationshaft directly connected to the first sun gear and the second ring gear,and selectively connected to the input shaft, a second rotation shaftdirectly connected to the first planet carrier and the second planetcarrier, and directly connected to the output shaft, a third rotationshaft directly connected to the first ring gear and selectivelyconnected to the input shaft or a transmission housing, a fourthrotation shaft directly connected to the second sun gear, andselectively connected to the input shaft or selectively connected to thetransmission housing, and a dog clutch selectively connecting the inputshaft to the fourth rotation shaft through two paths.

Each of the first and second planetary gear sets is a single pinionplanetary gear set.

The planetary gear train may further include a first transfer geardisposed between the input shaft and the first and third rotationshafts, a second transfer gear disposed on one path of the two paths,and a third transfer gear disposed on the other path of the two paths.

Gear ratios of the three transfer gears are different from each other.

The planetary gear train may further include a first clutch disposedbetween the first transfer gear and the first rotation shaft, a secondclutch disposed between the first transfer gear and the third rotationshaft, a first brake disposed between the third rotation shaft and thetransmission housing, and a second brake disposed between the fourthrotation shaft and the transmission housing.

The first planetary gear set is disposed at a radial exterior of thesecond planetary gear set.

The first, second, and third transfer gears, the first and secondclutches, and the second brake are disposed at one side of the first andsecond planetary gear sets, and the first brake is disposed a the otherside of the first planetary gear set.

The dog clutch is adapted to selectively transmit torque of the inputshaft to any one of the two paths or not to transmit either of the twopaths.

Other aspects and preferred embodiments of the invention are discussedinfra.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and may include hybridvehicles, electric vehicles, plug-in hybrid electric vehicles,hydrogen-powered vehicles and other alternative fuel vehicles (e.g.,fuels derived from resources other than petroleum). As referred toherein, a hybrid vehicle is a vehicle that may have two or more sourcesof power, for example both gasoline-powered and electric-poweredvehicles.

The methods and apparatuses of the present invention have other featuresand advantages which will be apparent from or are set forth in moredetail in the accompanying drawings, which are incorporated herein, andthe following Detailed Description, which together serve to explaincertain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

FIG. 2A is a drawing for illustrating operation of a dog clutch used inthe planetary gear train according to an exemplary embodiment of thepresent invention.

FIG. 2B is a drawing for illustrating operation of a dog clutch used inthe planetary gear train according to an exemplary embodiment of thepresent invention.

FIG. 2C is a drawing for illustrating operation of a dog clutch used inthe planetary gear train according to an exemplary embodiment of thepresent invention.

FIG. 3 is an operational chart of friction elements at each shift-speedapplied to the planetary gear train according to an exemplary embodimentof the present invention.

FIG. 4 is a lever diagram of the planetary gear train according to anexemplary embodiment of the present invention.

Reference numerals set forth in the Drawings include reference to thefollowing elements as further discussed below:

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of thepresent invention(s), examples of which are illustrated in theaccompanying drawings and described below. While the invention(s) willbe described in conjunction with exemplary embodiments, it will beunderstood that the present description is not intended to limit theinvention(s) to those exemplary embodiments. On the contrary, theinvention(s) is/are intended to cover not only the exemplaryembodiments, but also various alternatives, modifications, equivalentsand other embodiments, which may be included within the spirit and scopeof the invention as defined by the appended claims.

Hereinafter, an exemplary embodiment of the present invention willhereinafter be described with reference to the accompanying drawings sothat those skilled in the Field of the Invention to which the presentinvention pertains may carry out the exemplary embodiment.

Description of components that are not necessary for explaining thepresent exemplary embodiment will be omitted, and the same constituentelements are denoted by the same reference numerals in thisspecification.

In the detailed description, ordinal numbers are used for distinguishingconstituent elements having the same terms, and have no specificmeanings.

FIG. 1 is a schematic diagram of a planetary gear train according to anexemplary embodiment of the present invention.

Referring to FIG. 1, a planetary gear train according to an exemplaryembodiment of the present invention includes an input shaft IS receivingtorque of an engine, an output shaft OS disposed in parallel with andapart from the input shaft IS, first and second planetary gear sets PG1and PG2 disposed on the output shaft OS, three transfer gears TF1, TF2,and TF3, a dog clutch DC, and frictional elements including two clutchesC1 and C2 and two brakes B1 and B2.

Therefore, torque input from the input shaft IS is converted into eightforward speeds and one reverse speed by cooperation of the first andsecond planetary gear sets PG1 and PG2, and is then output through theoutput shaft OS.

The input shaft IS is an input member and receives torque from acrankshaft of the engine.

The output shaft OS drives a driving shaft including a driving wheelthrough a final reduction gear and a differential apparatus.

The first and second planetary gear sets PG1 and PG2 are disposed on theoutput shaft OS and are combined with each other to include fourrotation shafts N1, N2, N3, and N4.

The first planetary gear set PG1 is a single pinion planetary gear set,and includes a first sun gear 51, a first planet carrier PC 1 rotatablysupporting a first pinion P1 externally meshed with the first sun gearS1, and a first ring gear R1 internally meshed with the first pinion P1as rotation elements thereof.

The second planetary gear set PG2 is a single pinion planetary gear set,and includes a second sun gear S2, a second planet carrier PC2 rotatablysupporting a second pinion P2 externally meshed with the second sun gearS2, and a second ring gear R2 internally meshed with the second pinionP2 as rotation elements thereof.

The first planetary gear set PG1 is disposed at a radial exterior of thesecond planetary gear set PG2. The first sun gear S1 is integrallyformed with or is directly connected to the second ring gear R2 and thefirst planet carrier PC1 is directly connected to the second planetcarrier PC2 such that the first and second planetary gear sets PG1 andPG2 are operated as a compound planetary gear set including fourrotation shafts N1, N2, N3, and N4.

The first rotation shaft Ni is directly connected to the first sun gearSi and the second ring gear R2, and is selectively connected to theinput shaft IS.

The second rotation shaft N2 is directly connected to the first planetcarrier PC1 and the second planet carrier PC2, and is directly connectedto the output shaft OS to be always operated as an output element.

The third rotation shaft N3 is directly connected to the first ring gearR1, and is selectively connected to the input shaft IS or is selectivelyconnected to a transmission housing H.

The fourth rotation shaft N4 is directly connected to the second sungear S2, and is selectively connected to the input shaft IS through twopaths or is selectively connected to the transmission housing H.

The first, second, and third transfer gears TF1, TF2, and TF3respectively have first, second, and third transfer drive gears TF1 a,TF2 a, and TF3 a and first, second, and third transfer driven gears TF1b, TF2 b, and TF3 b externally meshed with each other.

In addition, gear ratios of the first, second and third transfer gearsTF1, TF2, and TF3 may be set according to target speed ratios. Forexample, the gear ratio of the second transfer gear TF2 is greater thanthat of the first transfer gear TF1 and is smaller than that of thethird transfer gear TF3.

Connections of the first, second, and third transfer gears TF1, TF2, andTF3 will be described in detail.

The first transfer gear TF1 connects the first rotation shaft N1 and thethird rotation shaft N3 to the input shaft IS. That is, the firsttransfer drive gear TF1 a is connected to the input shaft IS and thefirst transfer driven gear TF1 b is connected to the first rotationshaft N1 and the third rotation shaft N3.

The second transfer gear TF2 connects the fourth rotation shaft N4 tothe input shaft IS. That is, the second transfer drive gear TF2 a isconnected to the input shaft IS and the second transfer driven gear TF2b is connected to the fourth rotation shaft N4.

The third transfer gear TF3 connects the fourth rotation shaft N4 to theinput shaft IS. That is, the third transfer drive gear TF3 a isconnected to the input shaft IS and the third transfer driven gear TF3 bis connected to the fourth rotation shaft N4.

The dog clutch DC is widely used in a manual transmission and is wellknown to a person of an ordinary skill in the art. The dog clutch DCselectively transmits torque of the input shaft IS to the secondtransfer gear TF2 or the third transfer gear TF3, or does not transmitthe torque of the input shaft IS to either of the second and the thirdtransfer gears TF2 and TF3.

In addition, two clutches C1 and C2 connecting selected rotation shaftsto the input shaft IS and two brakes B1 and B2 connecting selectedrotation shafts to the transmission housing H are disposed as follows.

The first clutch C1 is disposed between the first transfer driven gearTF1 b of the first transfer gear TF1 and the first rotation shaft N1.

The second clutch C2 is disposed between the first transfer driven gearTF1 b of the first transfer gear TF1 and the third rotation shaft N3.

The first brake B1 is disposed between the third rotation shaft N3 andthe transmission housing H.

The second brake B2 is disposed between the fourth rotation shaft N4 andthe transmission housing H.

The frictional elements consisting of the first and second clutches C1and C2 and the first and second brakes B1 and B2 are conventionalmulti-plate friction elements of wet type that are operated by hydraulicpressure.

The first, second, and third transfer gears TF1, TF2, and TF3, the firstand second clutches C1 and C2, and the second brake B2 are disposed atone side of the first and second planetary gear sets PG1 and PG2, andthe first brake B1 is disposed at the other side of the first planetarygear set PG1.

FIGS. 2A-2C are drawings for illustrating operation of a dog clutch usedin the planetary gear train according to an exemplary embodiment of thepresent invention.

Referring to FIGS. 2A-2C, the dog clutch DC includes a dog clutch hubDCH, a dog clutch sleeve DCS, and first and second dog clutch gears DCG1and DCG2.

The dog clutch hub DCH is directly connected to the input shaft IS andcontinuously receives the torque of the input shaft IS.

The dog clutch sleeve DCS is splined to an exterior circumferentialportion of the dog clutch hub DCH to be slidable in an axial direction,and moves in the axial direction by an actuator.

The first and second dog clutch gears DCG1 and DCG2 are directlyconnected respectively to the third and second transfer gears TF3 andTF2. The first and second dog clutch gears DCG1 and DCG2 are selectivelyengaged with the dog clutch sleeve DCS and selectively receive thetorque of the input shaft IS.

Since the dog clutch DC is well known to a person of an ordinary skillin the art, detailed description thereof will be omitted.

FIG. 2A illustrates the dog clutch DC at a neutral state. As shown inFIG. 2A, the dog clutch sleeve DCS is positioned at the center and isnot engaged to neither of the first and second dog clutch gears DCG1 andDCG2 at the neutral state.

FIG. 2B illustrates that the torque of the input shaft IS is transmittedto the third transfer gear TF3. As shown in FIG. 2B, when the dog clutchsleeve DCS moves to the left in the drawings by the actuator, the dogclutch hub DCH and the first dog clutch gear DCG1 are operablyconnected.

In this case, the torque of the input shaft IS is transmitted to thethird transfer gear TF3 through the dog clutch DC.

FIG. 2C illustrates that the torque of the input shaft IS is transmittedto the second transfer gear TF2. As shown in FIG. 2C, when the dogclutch sleeve DCS moves to the right in the drawings by the actuator,the dog clutch hub DCH and the second dog clutch gear DCG2 are operablyconnected.

In this case, the torque of the input shaft IS is transmitted to thesecond transfer gear TF2 through the dog clutch DC.

FIG. 3 is an operational chart of friction elements at each shift-speedapplied to a planetary gear train according to an exemplary embodimentof the present invention.

Referring to FIG. 3, two frictional elements or one frictional elementand the dog clutch DC are operated at each shift-speed according to theexemplary embodiment of the present invention.

The first clutch C1 and the first brake B1 are operated and the dogclutch DC stays at the neutral state at a first forward speed 1ST.

The first clutch C1 and the second brake B2 are operated and the dogclutch DC stays at the neutral state at a second forward speed 2ND.

The first clutch C1 is operated and the dog clutch DC is operablyconnected to the third transfer gear TF3 at a third forward speed 3RD.

The first clutch C1 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a fourth forward speed 4TH.

The first clutch C1 and the second clutch C2 are operated and the dogclutch DC stays at the neutral state at a fifth forward speed 5TH.

The second clutch C2 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a sixth forward speed 6TH.

The second clutch C2 is operated and the dog clutch DC is operablyconnected to the third transfer gear TF3 at a seventh forward speed 7TH.

The second clutch C2 and the second brake B2 are operated and the dogclutch DC stays at the neutral state at an eighth forward speed 8TH.

The first brake B1 is operated and the dog clutch DC is operablyconnected to the second transfer gear TF2 at a reverse speed REV.

FIG. 4 is a lever diagram of the planetary gear train according to anexemplary embodiment of the present invention and illustrates shiftprocesses of the planetary gear train according to an exemplaryembodiment of the present invention by lever analysis method.

Referring to FIG. 4, four vertical lines of the first and secondplanetary gear sets PG1 and PG2 are set as the first, second, third, andfourth rotation shafts N1, N2, N3, and N4 from the left to the right. Inaddition, a lower horizontal line represents a rotation speed of “0”, anupper portion of the horizontal line represents positive rotation speed,and a lower portion of the horizontal line represents negative rotationspeed.

At this time, although rotation speed input to each rotation shaft isnegative rotation speed, for better comprehension and ease ofdescription, it is illustrates that positive rotation speed is input toeach rotation shaft.

Hereinafter, referring to FIG. 3 and FIG. 4, the shift processes of theplanetary gear train according to an exemplary embodiment of the presentinvention will be described in detail.

[First forward speed]: Referring to FIG. 3, the first clutch C1 and thefirst brake B1 are operated and the dog clutch DC stays at the neutralstate at the first forward speed 1ST.

As shown in FIG. 4, since the dog clutch DC stays at the neutral state,the torque of the input shaft IS is not input to the fourth rotationshaft N4 and is input to the first rotation shaft N1 through the firsttransfer gear TF1 by operation of the first clutch C1. In addition, thethird rotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a first shift line SP1 and D1 isoutput through the second rotation shaft N2 that is the output element.

[Second forward speed]: The first brake B1 that was operated at thefirst forward speed 1ST is released and the second brake B2 is operatedat the second forward speed 2ND.

In this case, since the dog clutch DC stays at the neutral state, thetorque of the input shaft IS is not input to the fourth rotation shaftN4 and is input to the first rotation shaft Ni through the firsttransfer gear TF1 by operation of the first clutch C1. In addition, thefourth rotation shaft N4 is stopped by operation of the second brake B2.

Therefore, the rotation shafts form a second shift line SP2 and D2 isoutput through the second rotation shaft N2 that is the output element.

[Third forward speed]: The second brake B2 that was operated at thesecond forward speed 2ND is released and the dog clutch DC is operablyconnected to the third transfer gear TF3 at the third forward speed 3RD.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 through the first transfer gear TF1 by operation ofthe first clutch C1 and is input to the fourth rotation shaft N4 throughthe third transfer gear TF3.

Therefore, the rotation shafts form a third shift line SP3 by the gearratios of the first transfer gear TF1 and the third transfer gear TF3,and D3 is output through the second rotation shaft N2 that is the outputelement.

[Fourth forward speed]: when the dog clutch DC is released from thethird transfer gear TF3 and is operably connected to the second transfergear TF2 at the third forward speed 3RD, the fourth forward speed 4TH isachieved.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 through the first transfer gear TF1 by operation ofthe first clutch C1 and is input to the fourth rotation shaft N4 throughthe second transfer gear TF2.

Therefore, the rotation shafts form a fourth shift line SP4 by the gearratios of the first transfer gear TF1 and the second transfer gear TF2,and D4 is output through the second rotation shaft N2 that is the outputelement.

[Fifth forward speed]: when the dog clutch DC returns to the neutralstate and the second clutch C2 is operated at the fourth forward speed4TH, the fifth forward speed 5TH is achieved.

In this case, the torque of the input shaft IS is input to the firstrotation shaft N1 and the third rotation shaft N3 through the firsttransfer gear TF1 by operation of the first clutch C1 and the secondclutch C2.

Therefore, the first and second planetary gear sets PG1 and PG2 becomedirect-coupling state, the rotation shafts form a fifth shift line SPS,and D5 is output through the second rotation shaft N2 that is the outputelement.

[Sixth forward speed]: The first clutch C1 that was operated at thefifth forward speed 5TH is released and the dog clutch DC is operablyconnected to the second transfer gear TF2 at the sixth forward speed6TH.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2 and is input to the fourth rotation shaft N4through the second transfer gear TF2.

Therefore, the rotation shafts form a sixth shift line SP6 by the gearratios of the first transfer gear TF 1 and the second transfer gear TF2,and D6 is output through the second rotation shaft N2 that is the outputelement.

[Seventh forward speed]: when the dog clutch DC is released from thesecond transfer gear TF2 and is operably connected to the third transfergear TF3 at the sixth forward speed 6TH, the seventh forward speed 7THis achieved.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2 and is input to the fourth rotation shaft N4through the third transfer gear TF3.

Therefore, the rotation shafts form a seventh shift line SP7 by the gearratios of the first transfer gear TF1 and the third transfer gear TF3,and D7 is output through the second rotation shaft N2 that is the outputelement.

[Eighth forward speed]: when the dog clutch DC returns to the neutralstate and the second brake B2 is operated at the seventh forward speed7TH, the eighth forward speed 8TH is achieved.

In this case, the torque of the input shaft IS is input to the thirdrotation shaft N3 through the first transfer gear TF1 by operation ofthe second clutch C2, and the fourth rotation shaft N4 is stopped byoperation of the second brake B2.

Therefore, the rotation shafts form an eighth shift line SP8 and D8 isoutput through the second rotation shaft N2 that is the output element.

[Reverse speed]: The first brake B1 is operated and the dog clutch DC isoperably connected to the second transfer gear TF2 at the reverse speedREV.

In this case, the torque of the input shaft IS is input to the fourthrotation shaft N4 through the second transfer gear TF2, and the thirdrotation shaft N3 is stopped by operation of the first brake B1.

Therefore, the rotation shafts form a reverse shift line RS and REV isoutput through the second rotation shaft N2 that is the output element.

The planetary gear train according to an exemplary embodiment of thepresent invention can achieve eight forward speeds and one reverse speedby combining two planetary gear sets PG1 and PG2 being the simpleplanetary gear sets with three transfer gears TF1, TF2, and TF3 beingthe externally-meshed gears, four frictional elements C1, C2, B1, andB2, and the dog clutch DC.

Since the first, second, and third transfer gears TF1, TF2, and TF3 andthe dog clutch DC are used, at least one planetary gear set and twofrictional elements may be removed, compared with the planetary geartrain. Therefore, manufacturing cost may be curtained by reducing thenumber of components.

In addition, optimum gear ratios may be set due to ease of changing gearratios by using the first, second, and third transfer gears TF1, TF2,and TF3 as well as the first and second planetary gear sets PG1 and PG2.Since the gear ratios can be changed according to target performance,starting performance, power delivery performance and fuel economy may beimproved.

In addition, two frictional elements or one frictional element and thedog clutch are operated at each shift-speed, and one frictional elementis released and another friction element is operated or only the dogclutch changes its operating state to shift to a neighboringshift-speed. Therefore, shift control condition is fully satisfied.

For convenience in explanation and accurate definition in the appendedclaims, the terms “upper”, “lower”, “inner” and “outer” are used todescribe features of the exemplary embodiments with reference to thepositions of such features as displayed in the figures.

The foregoing descriptions of specific exemplary embodiments of thepresent invention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteachings. The exemplary embodiments were chosen and described in orderto explain certain principles of the invention and their practicalapplication, to thereby enable others skilled in the art to make andutilize various exemplary embodiments of the present invention, as wellas various alternatives and modifications thereof. It is intended thatthe scope of the invention be defined by the Claims appended hereto andtheir equivalents.

What is claimed is:
 1. A planetary gear train of an automatictransmission for a vehicle comprising: an input shaft receiving torque;an output shaft disposed in parallel with and apart from the inputshaft; a first planetary gear set having three rotation elements; asecond planetary gear set having three rotation elements, wherein onerotation element is integrally formed with or is directly connected toone rotation element of the first planetary gear set; a first rotationshaft directly connected to the one rotation element of the firstplanetary gear set and/or the one rotation element of the secondplanetary gear set, and selectively connected to the input shaft; asecond rotation shaft directly connected to another rotation element ofthe first planetary gear set and another rotation element of the secondplanetary gear set, and directly connected to the output shaft; a thirdrotation shaft directly connected to the other rotation element of thefirst planetary gear set and selectively connected to the input shaft ora transmission housing; and a fourth rotation shaft directly connectedto the other rotation element of the second planetary gear set, andselectively connected to the input shaft through two paths orselectively connected to the transmission housing.
 2. The planetary geartrain of claim 1, further comprising three transfer gears disposedrespectively between the input shaft and the first and third rotationshafts and on the two paths.
 3. The planetary gear train of claim 1,further comprising a dog clutch selectively transmitting torque of theinput shaft to any one of the two paths or not transmitting either ofthe two paths.
 4. The planetary gear train of claim 1, wherein each ofthe first and second planetary gear sets is a single pinion planetarygear set.
 5. The planetary gear train of claim 4, wherein the firstrotation shaft is connected to a first sun gear of the first planetarygear set and a second ring gear of the second planetary gear set, thesecond rotation shaft is connected to a first planet carrier of thefirst planetary gear set and a second planet carrier of the secondplanetary gear set, the third rotation shaft is connected to a firstring gear of the first planetary gear set, and the fourth rotation shaftis connected to a second sun gear of the second planetary gear set. 6.The planetary gear train of claim 2, wherein the three transfer gearscomprises: a first transfer gear disposed between the input shaft andthe first and third rotation shafts; a second transfer gear disposed onone path of the two paths; and a third transfer gear disposed on theother path of the two paths.
 7. The planetary gear train of claim 2,wherein gear ratios of the three transfer gears are different from eachother.
 8. The planetary gear train of claim 6, further comprising: afirst clutch disposed between the first transfer gear and the firstrotation shaft; a second clutch disposed between the first transfer gearand the third rotation shaft; a first brake disposed between the thirdrotation shaft and the transmission housing; and a second brake disposedbetween the fourth rotation shaft and the transmission housing.
 9. Theplanetary gear train of claim 8, wherein the first planetary gear set isdisposed at a radial exterior of the second planetary gear set.
 10. Theplanetary gear train of claim 9, wherein the first, second, and thirdtransfer gears, the first and second clutches, and the second brake aredisposed at one side of the first and second planetary gear sets, andthe first brake is disposed at the other side of the first planetarygear set.
 11. A planetary gear train of an automatic transmission for avehicle comprising: an input shaft receiving torque; an output shaftdisposed in parallel with and apart from the input shaft; a firstplanetary gear set including a first sun gear, a first planet carrier,and a first ring gear as rotation elements thereof; a second planetarygear set including a second sun gear, a second planet carrier, a secondring gear as rotation elements thereof, wherein the first sun gear andthe second ring gear are integrally formed with or are directlyconnected to each other; a first rotation shaft directly connected tothe first sun gear and the second ring gear, and selectively connectedto the input shaft; a second rotation shaft directly connected to thefirst planet carrier and the second planet carrier, and directlyconnected to the output shaft; a third rotation shaft directly connectedto the first ring gear and selectively connected to the input shaft or atransmission housing; a fourth rotation shaft directly connected to thesecond sun gear, and selectively connected to the input shaft orselectively connected to the transmission housing; and a dog clutchselectively connecting the input shaft to the fourth rotation shaftthrough two paths.
 12. The planetary gear train of claim 11, whereineach of the first and second planetary gear sets is a single pinionplanetary gear set.
 13. The planetary gear train of claim 11, furthercomprising: a first transfer gear disposed between the input shaft andthe first and third rotation shafts; a second transfer gear disposed onone path of the two paths; and a third transfer gear disposed on theother path of the two paths.
 14. The planetary gear train of claim 13,wherein gear ratios of the three transfer gears are different from eachother.
 15. The planetary gear train of claim 13, further comprising: afirst clutch disposed between the first transfer gear and the firstrotation shaft; a second clutch disposed between the first transfer gearand the third rotation shaft; a first brake disposed between the thirdrotation shaft and the transmission housing; and a second brake disposedbetween the fourth rotation shaft and the transmission housing.
 16. Theplanetary gear train of claim 15, wherein the first planetary gear setis disposed at a radial exterior of the second planetary gear set. 17.The planetary gear train of claim 16, wherein the first, second, andthird transfer gears, the first and second clutches, and the secondbrake are disposed at one side of the first and second planetary gearsets, and the first brake is disposed a the other side of the firstplanetary gear set.
 18. The planetary gear train of claim 11, whereinthe dog clutch is adapted to selectively transmit torque of the inputshaft to any one of the two paths or not to transmit either of the twopaths.